Field of the Invention
This invention relates to sampling of a flowing stream of air or gas for aerosol particles. The invention relates more particularly to a device for drawing a small representative sample of air from a large-volume stream of laminar flow air in a clean room.
In clean rooms, for example, of the type useful for manufacture or handling of sterile pharmaceutical or pure chemical materials or for manufacture of sensitive optical or electronic parts or devices, it is required that the air be sampled periodically and routinely to establish the level of cleanliness in the room or at specified work stations or locations therein or when a particulate leak is suspected. A common practice is to draw this air sample from a length of tubing, the entrance of which is located generally in the area of interest. However, it is recognized that particles in laminar flow air streams in clean rooms move only along paths originating at the source of the particles and follow air flow stream lines through the clean room. Thus, a sampling tube collects a particle only if the entrance to the tube is placed directly in the path of the particle.
To solve this problem, more complex methods have been used. One process draws subsequent samples at a multitude of points, analyzes or counts these, and averages or combines the resulting data, but this method is difficult to execute and is time consuming and, thus, can be expensive. Additionally, phenomena which one may wish to measure occur over a shorter time period than this method requires, and therefore cannot be accurately represented by this technique. Another process uses a network of single sampling tubes interconnected by valves to allow selection of any one of the separate corresponding sampling locations at any one time. This is cumbersome and still difficult and time consuming to use. A process of drawing a number of air samples simultaneously, analyzing these simultaneously by means of an equal number of analyzing instruments, e.g. particle counters, and combining the resulting data is much faster than the methods above, but very cumbersome and possibly quite costly. One type of sampling device, specifically one type of an impactor, appears to draw and combine air samples simultaneously at a multitude of locations, the entrances to which are distributed over part of its exterior. This design is not, however, intended to achieve representative sampling of a body of air moving in laminar flow. The solid and continuous shape of the impactor causes the laminar flow of air to be diverted and to part around the device. Thus, the air brought to the multiplicity of sampling locations is representative only of that air in the streamline in front of the device which parts as it passes around the device.
Accordingly, it is an object of the present invention to provide a device for drawing a single continuous representative sample from a body of air moving in laminar flow for analysis or counting of the particles therein.